Switching device

ABSTRACT

The switching device has an anode and a cathode with a gasfilled space therebetween. Conduction causes ion implantation into the cathode, sputtering and adsorption pumping, with consequent reduction in gas pressure to result in cessation of conduction when the pressure decreases below the critical value. In the present switching device, this is prevented by providing an auxiliary gas volume which contributes gas as required.

"PATENIED JAN26 Ian SHEET 2 [1F 3 Gunter A.G. Hofmonn,

Ronald C. Knechrli,

INVENTORS.

ALLEN A. DICKE, Jr.,

AGENT.

SWITCHING DEVICE BACKGROUND This invention is directed to a switchingdevice of the crossed field type, employing Penning discharge, whereinregulation of the magnetic field strength controls electron path lengthso that this path length is above or below a critical value.

Switching devices of a general type are, known in the art. Penning US.Pat. No. 2,I82,736 describes such a switching device, while Boucher, etal. U.S. Pat. No. 3,2l5,893 and Boucher, US. Pat. No. 3,2l5,939 describeimprovements thereon. All three of these devices are primarily directedto rectifier type switching, and the Boucher and Boucher, ct al. patentsare directed to an improvement wherein the shape of the magnetic fieldimproves rectifying action by providing a lower breakdown voltage in onedirection than the other between the two electrodes which form thegas-filled space. However, these devices suffer from the problem thatthe ions are driven by the electric field toward the cathode and some ofthem are trapped there by ion implantation in the cathode material.Furthermore, these ions sputter cathode material which may produceadsorption pumping by the freshly sputtered material. These effectsresult in a reduction in gas pressure with consequent eventual loweringof this pressure below the critical value so that conduction ceases.Thus, these prior art structures are not capable of longtermconductivity. This problem is pointed out in the Penning patent.

With continually increasing electric power demands, there is increasedneed to exploit sources of power farther away from the users of largeamounts of electric power, with the consequent need for transporting theelectric power over greater distances. In the United States, a number ofour larger electric power-consuming areas are at some distance fromprimary power sources, such as sites for generation of hydroelectricpower, coal deposits and oil deposits. Accordingly, it becomes necessaryto transport electricity over greater distances. It is known that totransport high powers over long distances, DC can be economicallysuperior to AC. This has already led to a number of high power DCtransmission lines, such as the Pacific Intertie presently underconstruction between the Columbia River and Los Angeles. One limitationto the wide use of DC is the lack of practical high power DC switchingdevices. By making the continuous operation of a Penning dischargedevice possible over prolonged periods of time, the present inventionprovides means to make such a DC switch.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary fon'n that it is directed to aswitching device having first and second electrodes. The interelectrodespace is gas filled to such a pressure that the distance betweenelectrodes times the pressure is below a critical value when an electricfield is applied without a mag netic field. When a magnetic field abovea critical value is applied, the electrons move in a direction generallynormal to both the electric and magnetic fields. When the gas pressureexceeds a minimum valve, this results in ionizing collisions ofsufficient frequency to sustain a gas discharge capable of conducting asubstantial current between the electrodes. In order to maintain the gaspressure above this minimum valve, the device includes a gas supplywhich will maintain the gas pressure in the interelectrode space withinoperational limits. Preferably, the electrodes are tubular andconcentrically positioned, with the gas reservoir positioned interiorlyof the inner tubular electrode, with radial openings through the tubularinner electrode walls to connect the gas reservoir with theinterelectrode space.

Accordingly, it is an object of this invention to provide a switchingdevice of the crossed field type suitable for high current capacity andlong conduction periods. It is a further object to provide a switchingdevice which has a gas reservoir therein to maintain the gas at a properpressure. It is another object to provide a crossed field switch havinga concentric tubular anode and cathode with a gas reservoir interiorlyof the interior electrode. It is still another object to employ tubularelectrodes in a crossed field switching device, with the inner electrodebeing radially perforated and with the gas reservoir interiorly thereofso that the radial perforations provide for equalization of gas pressureand proper maintenance of gas pressure in the interelectrode space.Other objects and advantages of this invention will become apparent froma study of the following portion of this specification, theclaims, andthe attached drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1A is a schematic drawing of a portionof a power system of the nature in which a switch device of thisinvention is employed.

FIG. 1B is a schematic drawing of another portion of such a powersystem.

FIG. 2 is a perspective view, with parts broken away, of the switchingdevice of this invention.

FIG. 3 is a graph illustrating the Paschen curve.

DESCRIPTION The switching device is generally indicated at 10 in FIG. 2.Referring to FIGS. 1A and 1B, which illustrate the manner in which theswitching device 10 is employed in a circuit, two different applicationsof the switching device are indicated at 10A and 10B. In FIG. IA, powersource 12 drives generator 14. Power source l2 can be of anyconventional type, including hydroelectric, internal combustion engine,or steam, including nuclear heated steam. Generator 14 generatesalternating cur rent electricity of suitable voltage and frequency forthat portion of the system. It supplies alternating current transformer[6 which changes the voltage to one suitable for rectification anddirect transmission. When direct current is employed for economic, longdistance power transmission, this usually requires an increase involtage at the transformer output, as compared to its input. Transformerl6 supplies rectifier 9,

-which preferably includes a plurality of rectifiers arranged in bridgeform, depending on the plurality of phases at the output of transformerl6.

The rectifier in turn supplies transmission lines [8 and 20, throughswitch l0A. The presence of switch lOA, which can also serve as circuitbreaker in an appropriate circuit combination, permits the use ofuncontrolled rectifiers for the rectifier 9. This can lead tosubstantial savings over the use of controlled rectifiers such as arerequired by the present state of the art in the absence of a DC switchsuch as l0A. Transmission lines 18 and 20 are supported on a pluralityof towers 22 which support the lines in insulated fashion away from theterrain, from the area of generation to the area where the electricpower is to be employed. In some cases, transmission lines 18 and 20 maybe buried, and in some cases they will be underwater transmission lines.Furthermore, while two transmission lines are preferable so that thevoltage to ground can be divided between them, some systems may employ aground return, but such is not preferred for high-power systems.

Referring to FIG. lB, switch lOB is connected between transmission lines18 and 20 and load 24. While a simple switch and simple load areindicated, there are preferably two switches at ]0B, in order to switchthe power coming from each of transmission lines l8 and 20. Furthermore,load 24 may be a direct current load operating at transmission linevoltage, or it may be an inverter-transformer-load system. Switch l0B,with its load 24, illustrates the use of switch 108 for Map on thetransmission line. In the appropriate circuit combination, switch I08can also serve as a circuit breaker for the tap.

Referring to FIG. 2, the switch device 10 comprises housing 30 which iscarried upon bottom flange 32. Bottom flange 32 is in turn mounted uponbase flange 34, and they are secured together to provide a tight seal.Base flange 34 stands upon foot 36 for supporting the switch devicestructure. Furthermore, vacuum connection 38 is connected to base flange34 for drawing a suitable vacuum on the interior of housing 30 and thenletting into the tube the desired gas (e.g. hydrogen, including itsisotope deuterium) at the required pressure. Housing 30, together withbottom flange 34, serves as a suitable vacuum tight envelope.

Cathode 40 is in the form of a cylindrical tube. It is spaced inwardlyfrom housing 30. Cathode 40 has a lower cap 42 by which it is supportedfrom base flange 34 by means of standoff 44. Lower cap 42 does not needto effect closure, but simply provides mechanical support for thecathode and reduces plasma end losses. By this construction, the entirecathode can be withdrawn through the large opening in bottom flange 32when the flanges are separated for inspection and service of the cathodeand inspection and service of the interior of housing 30. Cathode 40 ismetallic and can be made of stainless steel. The cathode is connected tothe foot 36 such as by a metallic strip. Thus, foot 36 provides one ofthe electrical connections to the switching device l0. Cathode 40preferably has an axial slot to prevent the circumferential circulationof current during switching transients, when the axial magnetic fieldchanges with time.

Anode 46 is of cylindrical tubular construction and is positionedconcentrically with cathode 40 to provide a radial space therebetweenhaving the dimension d. The'radial space d is substantially equal at allfacing positions of the anode and cathode. Housing 30 has a top cap 48upon which anode 46 is positioned. The anode is maintained in positionby employing anode cap 50 which is secured to the cylindrical anode 46,and in turn carries mounting stud 52. Mounting stud 52 provides bothmechanical support by being secured to housing cap 48, and provideselectrical continuity through the cap by connector 54. Preferably, anodecap 50 is spaced below top cap 48 and connector 54 passes throughinsulative mounting stud 52 so that connector 54 and the entire anodeare electrically separated from the housing. Alternatively, top cap 48can be of insulative material.

Anode 46 has a plurality of holes 56 therethrough so that the interiorspace within anode 46 is in communication with the interelectrode space.The volume within the interior of anode 46 is preferably in the order ofl times the volume in the interelectrode space. Magnet 58 is positionedon the exterior of housing in such a manner as to provide magnetic linesof force in the interelectrode space which are substantially parallel tothe axis of the electrodes of switching device l0 over at least part ofthe electrode length. Magnet 58 is illustrated as being anelectromagnet, and such is preferred so that the magnetic field canreadily be switched on and off. The power supply to magnet 58 ispreferably of such nature as to provide for rapid turn on and off of thefield. lts strength is such as to provide a field between 25 and ISOGauss; 70 Gauss was found to be a preferred value for the dimensionsgiven below used in our experiments to date, considering the turn on andturn off effects, as well as magnet power consumption.

The interior of anode 46, as well as the interelectrode space, is filledwith a gas to an appropriate pressure. Referring to FIG. 3 the Paschencurve is shown therein. This curve illustrates that at a certaincritical product of the interelectrode pressure p times theinterelectrode spacing d, the voltage to breakdown is fairly low. Italso illustrates at point A that for a lower product, voltage to causebreakdown is considerably higher. This is because at lower pressure, theelectron meanfree path exceeds the interelectrode spacing d, and theionization rate decreases, which makes it more difficult to sustain thedischarge and makes it possible to withstand higher voltage betweenelectrodes before breakdown occurs.

When the magnetic field. is off, electron flow is only under theinfluence of the electric field from the cathode to the anode so thatthe average electron path length is substantially equal to theinterelectrode space d, and is less than the electron mean-free pathlength. Thus, there is no sustained ionization, electron flow is low,and the switching device can withstand a high standoff voltage, for itsoperating point lies approximately below point A on the Paschen curve.However, when the magnetic field is applied to the interelectrode spaceby electromagnet 58, the axial magnetic field causes the mean-freeelectron path to be circular until a collision occurs. in this longerpath caused by the magnetic field effect, there are sufficientcollisions to maintain ionization because the mean-free electron pathlength is sufficiently longer than the interelectrode spacing d. Thus,so long as a sufficient magnetic field is applied, once electrons startflowing, the flow is maintained until the magnetic field is cut off.When cut off, the electrons again flow radially so that ionization isnot maintained.

Since the net electron flow is from the cathode to the anode, and flowof electrons through the interelectrode space results in collisions withgas atoms to cause ionization, a certain number of these ionizingcollisions cause the ions to be driven into the surface of the cathode.Gas pumping by ion implantation and by adsorption on freshly sputteredmaterial occurs with the result that the amount of ionized and neutralgas decreases after the switching device has been conducting for aperiod of time to a point where conduction cannot be maintained. Thiscauses unwanted or premature off switching of the device, when the onlygas available is that in the interelectrode space. in the presentswitching device, holes 56 in anode 46 permit the space interiorly ofanode 46 to communicate with the interelectrode space. Thus, the gaswithin the interior of anode equalizes pressure with the gas in theinterelectrode space through holes 56. in order to realize short-timeconstants, holes are necessary rather than attempting equalizationaround the ends of the anode tube.

In a particular example, the interelectrode radial distance d is aboutl5 millimeters, with an anode diameter of millimeters and axial lengthof 300 millimeters. Holes 56 are present in such quantity and size as toprovide about 30 percent of overall anode area as communication space.With such dimensions, and with a switching device 10 capable of holdingoff 25 kilovolts, normal gas pressure in the interelectrode space and inthe anode interior, when the switching device 10 is new, is about 0.04millimeters of mercury. Hydrogen is the preferred gas, including itsisotope deuterium, in the switch of this example. if it were not for thegas within the interior of the and anode being in communication with theinterelectrode space a charge of 0.4 Coulombs is approximatelysufficient to reducethe interelectrode space gas pressure to a pointwhere the switching device will off switch, due to gas loss. However,with the indicated interior anode volume available to the interelectrodespace, a charge greater than 2.4 Coulombs can be passed before the gaspressure decreases sufficiently to cause danger of off switching. Theconduction period can be further extended by providing an auxiliary gassource such as titanium hydride ribbon or sponge 60 at an appropriatetemperature, inside of the anode volume or in communication with it.Titanium metal in such a form that it has a large surface area, forexample as ribbon or sponge, has the property that, at elevatedtemperature and in a hydrogen atmosphere, great quantities of hydrogenare absorbed. While maintaining an appropriate temperature, a reductionin the hydrogen pressure therearound results in discharge of hydrogenfrom the metal in an attempt to maintain an equilibrium pressure. Suchlarge quantities of hydrogen can be absorbed that, under the pressureand volume considerations of the structure of this invention, thetitanium hydride source functions virtually as an infinite source tomaintain the equilibrium pressure. it should, however, be noted thatsuch an auxiliary gas source alone, without the holes providing fastcommunication for the gas between the interelectrode volume and thegas-filled volume inside the anode, would not suffice to preventself-interruption of the gas discharge due to gas depletion in theinterelectrode space under passage of a high current. Under suchconditions, the time constant of a conventional auxiliary gas sourcewould be too long compared to the gas depletion time constant of theinterelectrode space. In this example, a magnetic field in the order of70 Gauss is provided in the interelectrode space. The holes 56 in theanode do not limit current-carrying capacity, because discharge iscathode area limited, rather than anode area limited.

With the dimensions illustrated, the switching device I0 is capable ofoff switching DC loads of L000 amperes, and hold off 25 kilovolts withina recovery time on the order of about 25 microseconds. Thus, it isuseful as a DC switch or element ofa DC circuit breaker, as illustratedin FIGS. lA and IB.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof'the inventive faculty. Accordingly. the scope of this invention isdefined by the scope of the following claims.

We claim:

1. A switching device, said switching device comprising:

An envelope arranged to maintain a subatmospherie pressure within saidenvelope, gas at a subatmospheric pressure within said envelope;

a cathode electrode within said envelope, said cathode electrode havingan active cathode face;

an anode electrode positioned within said envelope, said anode electrodehaving an active anode face facing said active cathode face, anddefining an interelectrode space;

electrical connections on said anode electrode and said cathodeelectrode, said gas pressure within said envelope in said interelectrodespace being sufficiently low so that, upon application of a directvoltage between said anode and said cathode to produce an electric fieldtherebetween, there is substantially no electric current conductiontherebetween in the absence of a magnetic field;

an electromagnet on said envelope, said electromagnet.

when energized, supplying a magnetic field in the interelectrode spaceat an angle with respect to the electric field so that, when theelectromagnet is energized, ionization takes place in the interelectrodespace to permit electric current to flow between said anode and saidcathode and, upon cessation of the magnetic field, ionization ceases sothat there is substantially no electric current conduction between saidanode and said cathode so that control of the magnetic field switchesthe electric current, the improvement comprising:

perforations in at least one of said electrodes, a gas space on theother side of said perforated electrode opposite said active electrodeface so that gas can communicate through said perforations into saidinterelectrode space. said gas space having a greater volume than saidinterelectrode space.

2. The switching-device of claim I wherein said anode elec trode isperforated and said gas space is on the opposite side of said anodeelectrode from the active face thereof.

3. The switching device of claim 2 wherein said cathode is tubular andsaid anode is tubular and said anode is positioned within said tubularcathode so that the space between said anode and said cathode issubstantially uniform.

4. The switch of claim 3 wherein said anode and said cathode are eachcylindrical tubes, said cylindrical tubes of said anode and said cathodeeach having an axis, said axes being substantially coincident, theinterelectrode space being measured in a distance perpendicular to saidaxes, and the electric field being applied in a direction perpendicularto said axes, said magnetic field being applied in a directionsubstantially parallel to said axes.

5. The switching device of claim I wherein an auxiliary gas source ispositioned on the side of said perforated electrode opposite said activeelectrode face.

6. The switching device of claim 5 wherein the gas is hydrogen and saidauxiliary gas source is a metallic hydride.

1. A switching device, said switching device comprising: An envelopearranged to maintain a subatmospheric pressure within said envelope, gasat a subatmospheric pressure within said envelope; a cathode electrodewithin said envelope, said cathode electrode having an active cathodeface; an anode electrode positioned within said envelope, said anodeelectrode having an active anode face facing said active cathode face,and defining an interelectrode space; electrical connections on saidanode electrode and said cathode electrode, said gas pressure withinsaid envelope in said interelectrode space being sufficiently low sothat, upon application of a direct voltage between said anode and saidcathode to produce an electric field therebetween, there issubstantially no electric current conduction therebetween in the absenceof a magnetic field; an electromagnet on said envelope, saidelectromagnet, when energized, supplying a magnetic field in theinterelectrode space at an angle with respect to the electRic field sothat, when the electromagnet is energized, ionization takes place in theinterelectrode space to permit electric current to flow between saidanode and said cathode and, upon cessation of the magnetic field,ionization ceases so that there is substantially no electric currentconduction between said anode and said cathode so that control of themagnetic field switches the electric current, the improvementcomprising: perforations in at least one of said electrodes, a gas spaceon the other side of said perforated electrode opposite said activeelectrode face so that gas can communicate through said perforationsinto said interelectrode space, said gas space having a greater volumethan said interelectrode space.
 2. The switching device of claim lwherein said anode electrode is perforated and said gas space is on theopposite side of said anode electrode from the active face thereof. 3.The switching device of claim 2 wherein said cathode is tubular and saidanode is tubular and said anode is positioned within said tubularcathode so that the space between said anode and said cathode issubstantially uniform.
 4. The switch of claim 3 wherein said anode andsaid cathode are each cylindrical tubes, said cylindrical tubes of saidanode and said cathode each having an axis, said axes beingsubstantially coincident, the interelectrode space being measured in adistance perpendicular to said axes, and the electric field beingapplied in a direction perpendicular to said axes, said magnetic fieldbeing applied in a direction substantially parallel to said axes.
 5. Theswitching device of claim l wherein an auxiliary gas source ispositioned on the side of said perforated electrode opposite said activeelectrode face.
 6. The switching device of claim 5 wherein the gas ishydrogen and said auxiliary gas source is a metallic hydride.